Electrodeless discharge lamps are described in sources such as U.S. Pat. No. 4,010,400 to Hollister, M.H. incorporated herein by reference, which describes an electrodeless discharge lamp including an induction coil positioned in a central cavity surrounded by a sealed discharge vessel. The discharge vessel contains a mixture of a metal vapor and an ionizable gas. Mercury vapor and argon are frequently used. The induction coil is connected in series with a capacitor. A radio frequency (RF) signal is generated by an oscillator, amplified and fed into a series L-C network. When the L-C network is energized by this RF signal, it resonates and the induction coil generates electromagnetic energy which is transferred to the gaseous mixture in the sealed discharge vessel.
Electrodeless discharge lamps operate in two stages. In the start-up electromagnetic discharge mode, i.e., as the lamp is being turned on, the electric field from the induction coil causes some of the atoms in the gaseous mixture to be ionized. The electrons which are freed in this process circulate around the induction coil within the sealed discharge vessel. Collisions between these electrons and the atoms release additional electrons until a plasma of circulating charged particles is formed. The induction coil and plasma behave in the manner similar to a transformer, i.e. with the coil acting as the primary winding and the discharge current acting as the secondary winding. However, because of air gaps between the coil and the sealed discharge vessel, typically made of glass, the magnetic coupling between the coil and the gaseous mixture is normally quite poor.
Many of these collisions excite the mercury atoms to a higher energy state rather than ionizing them. As the mercury atoms fall back from the higher energy state, they emit radiation, primarily in the ultraviolet (UV) spectrum. This radiation impinges on phosphors which coat the inner surface of the discharge vessel. The phosphors are selected to be highly excitable by UV radiation and in turn emit visible light as they return from their excited state.
During the second stage of operation, i.e. after the electron flow in the gaseous mixture has been established, the magnetic field generated by the induction coil becomes of primary importance in maintaining the discharge.
The early introduction of the incandescent lamp caused a major revolution in the way light was delivered. Originally, the pear-shaped glass vessel was chosen as the enclosure of choice because it was strong, inexpensive, and was the easiest shape for a glass blower to achieve. The vessels were then produced manually by blowing air into a bit of molten glass at the end of a long pipe. Although glass vessels are mass produced today using modern machinery, the pear-shaped vessel has been retained since the configuration also lends itself to high speed machines. As a result, the pear shape has become the industry standard.
The incandescent lamp in a pear-shaped vessel has several drawbacks. Being a point source of light, it causes an unpleasant glare which requires the addition of shades, reflectors, and/or baffles to make the lighting system more acceptable to the user. Unfortunately, these techniques also reduce the energy efficiency of the light source. Various glass shapes have been used to deliver an improved quantity of usable light for particular applications. This includes the pressed glass reflector (known as a PAR lamp) which delivers light in a preferential direction, making it more efficient for task and display lighting applications.
Electrodeless discharge lamps have generally retained the original pear-shape because these newer light sources were intended to serve as energy efficient replacements for the standard incandescent lamp. Since the existing sockets had been designed around the standard industry bulb shape, it was important to retain compatibility with the existing physical shape.
Task lighting or directional applications present a challenge for electrodeless discharge lamps. Electrode-less discharge lamps have the characteristics of providing a uniform level of illumination over the surface area of the phosphor layer, i.e. they are not point sources of light. Therefore, reflectors are not particularly efficient in increasing the light level in any preferential direction.
Accordingly, there is a need for electrodeless discharge lamps which deliver useful quantities of light in a preferential manner to efficiently illuminate a task or specific area.